That's what I was talking about with an electrician doing a trace of lines coming out of your box. He can confirm they all go to your unit, or he'll tell you one or more are going somewhere else. Flipping breakers on and off to see if your neighbors lights go out is fine, but what if they don't have a light on that circuit? An electrician doing a trace will be able to give you a good analysis.
Unless the neighbour has a talk with the electrician before he gives his diagnostic....
Back to square 1.......